Using acoustic backscatter data to provide estimates of profiles of suspended sediment concentrations for cohesive particles in the field
Permanent link to Research Commons versionhttps://hdl.handle.net/10289/16201
Many estuaries and rivers in New Zealand and other locations worldwide are stressed by the input of fine (muddy) sediments. Suspended sediment concentrations (SSC) are often used as a key indicator of water quality and are a critical variable in management practices to aid in the health of estuarine habitats. However, robust measurements of SSC are difficult to obtain, owing in part to large gradients in conditions, and the process of ‘flocculation’ or aggregation of fine particles. Amongst the range of measurement tools available, acoustic backscatter (ABS) sensors offer significant potential to provide temporally and spatially well resolved profiles of SSC from the field. The process of ‘inversion’ or converting the acoustic backscatter signal into SSC is effective when all terms are known within the theoretical context. However, the challenge facing the use of ABS technology in the field, is the scattering properties, the form function f∞ and the total scattering cross-section Σₛ, of cohesive particles typical in estuarine environments, are unknown. For this study, a unique laboratory calibration combined with ABS field data from a mudflat, were used to investigate if a pair of f∞ and Σₛ values could be evaluated for finer grain, mud material as opposed to sand particles. The derived f∞ and Σₛ values appeared to be influenced by other physical controls from the field environment and were sensitive to several assumptions made in the inversion equation. Nonetheless, the acoustic data yielded reasonable estimates of SSC in a field setting, when compared to discrete single-point measurements of SSC obtained from a calibrated optical backscatter sensor. However, there were also times when the inversion methodology did not work well (particularly for acoustic transducers with lower frequencies). Moreover, inversion results were better when using time-varying values for f∞ and Σₛ, the inversion procedure implemented with a universal set parameters did not perform as well. These results imply more research is needed in gathering better descriptions of the f∞ and Σₛ values from multiple ABS frequencies, to improve inversion SSC results from the field.
The University of Waikato
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